Whether Deckard is himself a replicant is left deliberately ambiguous.
In the original novel, a security force exists in addition to the regular police force in Los Angeles, whose operatives are termed 'blade runners' and who are actually replicants, created to do that special job of locating and terminating replicants that go 'rogue'. It's a kind of 'set a thief to catch a thief' arrangement.
This force of replicants did not make it into the movie; it is one of several elements in the novel which were lost in the process of adapting the original story for the screen.
But the notion may have persisted, in the mind of the director, that 'blade runner' is a job carried out by replicants.
He hints at this in the movie, by including - in the 2007 director's Final Cut - the Unicorn scene, implying that Deckard is having artificial dreams, one of the hallmarks of a replicant (an element which echos the original title of the novel: 'Do Androids Dream of Electric Sheep?')
Also, Deckard treats the replicants very badly, executing all of them (except Rachael) - thus showing himself to be less human than they. For when the opportunity arises for a replicant to kill Deckard, the one called Roy saves his life. Morally, this implicitly elevates the replicants over Deckard.
Yet there is no indication in the movie of Deckard having a limited lifespan, something which all the replicants suffer from, beyond the fact that the police allow him and Rachael to ride away into the sunset together, unmolested, at the end of the picture; thereby implying that the police believe the couple have too little time left to cause any further problem.